part b: non-imaging scintillation detectors unit ii: nuclear medicine measuring devices lectures 7...
TRANSCRIPT
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Part B: Non-imaging
Scintillation Detectors
Unit II: Nuclear Medicine Measuring Devices
Lectures 7 & 8
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ObjectivesObjectives Define scintillationDefine scintillation Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s
proportional response to deposited gamma energyproportional response to deposited gamma energy Describe the components of a photomultiplier tube and their functionDescribe the components of a photomultiplier tube and their function Discuss the purpose of other associated electronics within the Discuss the purpose of other associated electronics within the
scintillation detectorscintillation detector Describe the calibration process for single and multi-channel Describe the calibration process for single and multi-channel
analyzersanalyzers Discuss peak broadening and the determination of a percent energy Discuss peak broadening and the determination of a percent energy
windowwindow Calculate percent energy resolution from FWHM and its importance in Calculate percent energy resolution from FWHM and its importance in
quality controlquality control Describe quality control tests for scintillation detectors and their Describe quality control tests for scintillation detectors and their
required frequencyrequired frequency
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http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 13.
Scintillation Process in NaI(Tl)Scintillation Process in NaI(Tl)
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Gamma Photon
Excited Electrons
http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
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http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
Visible light
(350-500nm λ)
Returning electrons
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http://www.webelements.com/webelements/compounds/media/Na/I1Na1-7681825.jpg
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Gamma Photon
Excited Electrons
http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
NaI at Room Temperature
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Excited Electrons Don’t Fluoresce
http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
NaI at Room Temperature
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http://www.webelements.com/webelements/compounds/media/Na/I1Na1-7681825.jpg
Thallium
So we add an impurity – a big ole ugly Thallium Atom
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NaI at Room Temperature
Thallium
Thallium forms a luminescent Center in the gap that catches excited electrons
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NaI at Room Temperature
Thallium
When electrons return to the valence band from the gap, they give off light
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http://www.webelements.com/webelements/compounds/media/Na/I1Na1-7681825.jpg
Thallium
Therefore, we say our crystal of Sodium Iodide is Thallium Activated
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Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.
Gamma Photon
NaI (Tl) Crystal (hermetically sealed in reflective material)
Visible light
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140 keV Gamma Photon
Visible light produced is 325 to 550 nm wavelength
For each keV of gamma photon energy absorbed, 20-30 visible light photons are released by the crystal.
Therefore, a 140 keV photon will cause about 3000 visible light photons to be released from the crystal
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Important Magic to RememberImportant Magic to RememberThe higher the gamma photon energy :
The more visible photons created
The visible light emitted from the NaI (Tl) crystal is PROPORTIONAL to the incident energy of the gamma photon.
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Timing after a gamma interaction:
Scintillation peak in about 30 nsec and about 2/3 of light emitted after 230 nsec
At lower rates of interaction (low count rate), a scintillation event typically ends before the next
Hence scintillation detectors operate in pulse mode
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Photomultiplier TubePhotomultiplier Tube
http://www.kolumbus.fi/michael.fletcher/pmt_1.jpg
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Photomultiplier tubes coupled to NaI(Tl) Crystals
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Crystal/PMT InterfaceCrystal/PMT Interface
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 101.
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CsSb CsSb PhotocathodePhotocathode
NaI (Tl) NaI (Tl) CrystalCrystal
Gamma PhotonGamma Photon
Visible LightVisible Light
Emitted Emitted electrons from electrons from photocathodephotocathode
Optical Window Optical Window (transparent (transparent
material)material)
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Quantum Efficiency: Quantum Efficiency: A measure of A measure of how well a how well a photoemissivephotoemissive material material emits electrons when exposed to emits electrons when exposed to
various wavelengths of lightvarious wavelengths of light
The above graph The above graph shows this shows this
photoemissive photoemissive material is most material is most
productive at 400 nmproductive at 400 nm—about the same —about the same
wavelength created wavelength created by NaI (Tl) by NaI (Tl) scintillationscintillation
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 102.
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For every 3 – 5 visible light photons reaching the photocathode, For every 3 – 5 visible light photons reaching the photocathode, 1 electron is emitted1 electron is emitted
Proportionality MaintainedProportionality Maintained
Note: This is Note: This is the least the least
efficient phase efficient phase of the transferof the transfer
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Photomultiplier Tube (PMT) ConstructionPhotomultiplier Tube (PMT) Construction
Focusing Grid – Guides in ElectronsFocusing Grid – Guides in Electrons
Dynodes – Usually positively charged Dynodes – Usually positively charged photoemissive coated electrodes with photoemissive coated electrodes with
increasing voltagesincreasing voltages
Anode – end positively chargedAnode – end positively charged
High voltage power supply – needed to High voltage power supply – needed to increase incrementally the potential increase incrementally the potential
difference between dynodesdifference between dynodes
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Increased voltages between dynodes : Increased voltages between dynodes : means increased KE of electrons means increased KE of electrons
between dynodesbetween dynodes
Increased KE of electrons mean that Increased KE of electrons mean that more electrons are knocked off at each more electrons are knocked off at each
dynode (3X to 6X at each)dynode (3X to 6X at each)
At 6X each with 10 dynodes means 6At 6X each with 10 dynodes means 610 10
electrons produced (about 60 million)electrons produced (about 60 million)
(Still small – 1 Amp = 1 C/s; (Still small – 1 Amp = 1 C/s; 1 C = 6.3 X 101 C = 6.3 X 1018 18 electrons)electrons)
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Millions of electrons are produced by Millions of electrons are produced by the dynodes the dynodes in direct responsein direct response to the to the
initial few electrons emitted by the initial few electrons emitted by the photocathode photocathode in direct responsein direct response to the to the
visible light photons emitted by the visible light photons emitted by the NaI(Tl) crystal NaI(Tl) crystal in direct responsein direct response to the to the
energy level of the gamma photon energy level of the gamma photon interacting with the crystal interacting with the crystal
End result:End result:
Increased gamma energy : means Increased gamma energy : means increased electrons reaching the anode increased electrons reaching the anode
at the end of the tubeat the end of the tube
This means that the height of the This means that the height of the electric pulse created by the millions of electric pulse created by the millions of
electrons at the anode will be an electrons at the anode will be an indicator of gamma energy level indicator of gamma energy level
Proportionality MaintainedProportionality Maintained
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Inefficiencies from the transfer Inefficiencies from the transfer of energyof energy
The proportionality of the The proportionality of the system is approximate and system is approximate and contains random statistical errorcontains random statistical errorSee Table 2-1 (p. 21)See Table 2-1 (p. 21)
Note that Tc-99m gamma produces Note that Tc-99m gamma produces less “information carriers” than the less “information carriers” than the higher energy gamma from Cs-137higher energy gamma from Cs-137
Therefore, Cs-137 has better Therefore, Cs-137 has better counting statistics and less variation counting statistics and less variation in the heights of its pulsesin the heights of its pulses
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Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60.
From the PMT the signal goes from the anode to the preampFrom the PMT the signal goes from the anode to the preamp
PreamplifierPreamplifier
Increases pulse Increases pulse 4X to 5X4X to 5X
Matches Matches impedanceimpedance to the to the system’s circuitrysystem’s circuitry
Proportionality MaintainedProportionality Maintained
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Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60.
Next the signal goes from the preamp to the ampNext the signal goes from the preamp to the amp
AmplifierAmplifier
Pulse undergoes: Pulse undergoes:
1. 1. Pulse ShapingPulse Shaping
2.2.Linear AmplificationLinear Amplification
(Amplified 1 to 100 X(Amplified 1 to 100 X
by Gain control)by Gain control)
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Pulse Shaping (Amplifier)Pulse Shaping (Amplifier)
From Sodee:From Sodee:
““Change to voltage Change to voltage converter that increases converter that increases the signal-to-noise ratio.”the signal-to-noise ratio.”
Makes “splat” of voltage Makes “splat” of voltage pulse into a “pop.”pulse into a “pop.”
Increases count rate Increases count rate capability of the systemcapability of the system
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 138.
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Pulse ShapingPulse Shaping
Sorenson, p. 88Sorenson, p. 88
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Pulse Shaping: RC Circuits and Pulse Shaping: RC Circuits and Noise EliminationNoise Elimination
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Linear Amplification (Amplifier)Linear Amplification (Amplifier)P
ulse
Vol
tage
Pul
se V
olta
ge
TimeTime
Each of these pulses Each of these pulses represents a different gamma represents a different gamma photon energy detected by the photon energy detected by the NaI (Tl) crystal.NaI (Tl) crystal.
We want to preserve this We want to preserve this proportionality in the pulses for proportionality in the pulses for it represents the proportional it represents the proportional differences in the gamma differences in the gamma energies detected.energies detected.
But we need a stronger signal But we need a stronger signal to work with.to work with.
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Linear Amplification (Amplifier)Linear Amplification (Amplifier)P
ulse
Vol
tage
Pul
se V
olta
ge
TimeTime
The linear amplifier amplifies The linear amplifier amplifies all the pulses proportionally.all the pulses proportionally.
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Linear Amplification (Amplifier)Linear Amplification (Amplifier)P
ulse
Vol
tage
Pul
se V
olta
ge
TimeTime
Proportionality Proportionality MaintainedMaintained
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Linear Amplification (Amplifier)Linear Amplification (Amplifier)P
ulse
Vol
tage
Pul
se V
olta
ge
TimeTime
Proportionality Proportionality MaintainedMaintained
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Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60.
From Sodee…From Sodee…
“The pulse height is directly proportional to the energy of the incident gamma photon.”
Prekeges, J.
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Linear Amplification (Amplifier)Linear Amplification (Amplifier)P
ulse
Vol
tage
Pul
se V
olta
ge
TimeTime
The linear amplifier amplifies The linear amplifier amplifies all the pulses proportionally.all the pulses proportionally.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
GAIN
1
2 5
10
We can designate (calibrate) the height of our pulses by
Gain Control
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
A new set of pulses from photons with the following energies…230
keV80 keV
120 keV140 keV
30 keV
180 keV
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
10
2
0
30
4
0
5
0
60
(We have magic eyes and know what these energies are before our detecting system does.)
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
We introduce a linear voltage scale…
230 keV80 keV
120 keV140 keV
30 keV
180 keV
For Comparision,
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
We introduce our gain control voltage of our amplifier to increase the voltage pulse heights associated with photons of the given energies
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10At gain setting “1” we see the pulse voltages below
And they appear on our voltage scale as follows…(Photon energy represented by color points
only)
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
Changing the gain to “2” doubles the size of our pulses.
And Shifts our energy points lineup to the right
GAIN
1
2 5
10
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
To amplify our pulses even more, we’ll need to change the scale of our pulse voltages so they will fit on our slide.
GAIN
1
2 5
10
10
2
0
30
4
0
5
0
60
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
GAIN
1
2 5
10
50
75
1
00
1
25
1
50
1
75
2
00
22
5
Now that our scale’s adjusted on our graph, we can really start cranking up the gain and see its
effects.
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GAIN
1
2 5
10
Let’s see what happens when we crank this puppy up to “5.” (5 X the gain!)
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
GAIN
1
2 5
10
50
75
1
00
1
25
1
50
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2
00
22
5GAIN
1
2 5
10
(Notice how these photon energy points spread to the right even more.)
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GAIN
1
2 5
10
Let’s crank the gain up to “10.” (10 X amplified!!)
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
GAIN
1
2 5
10
50
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1
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5GAIN
1
2 5
10
(-Notice how these points representing the different photon energies now line up with our voltage scale?)
GAIN
1
2 5
10
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230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
GAIN
1
2 5
10
GAIN
1
2 5
10
GAIN
1
2 5
10
GAIN
1
2 5
10With our gain at 10, we’ve made our photon energies register to voltage numbers that equal the keV for each photon.
Because we knew our photon energies before-hand, we can therefore say that we have “calibrated” our voltage scale so that each additional volt means an additional keV of photon energy.
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GAIN
1
2 5
10
32
16
8
42
4
2
1
0.5
0.25
Special note: not all gain scales read the same.
Some are “inverse gain” scales and can be considered as representing the denominators of fractions.
For example:
Going from an inverse gain setting of 32 to 16 is like adjusting the voltage control from 1/32 to 1/16.
This still in effect doubles the voltage response of the pulse.
Inverse gain scales
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
50
75
1
00
1
25
1
50
1
75
2
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22
5
We have collected these six photons of various energies over this given time period…
…and have appointed them each a place on our voltage scale (which now represents 1 keV per volt).
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Pul
se V
olta
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ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
50
75
1
00
1
25
1
50
1
75
2
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22
5
Now,
What if we let the clock run and keep on detecting more photons?
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Pul
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Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
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We’d get a random mixed bag of photons.
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Pul
se V
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geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
50
75
1
00
1
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1
50
1
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2
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5
For the sake of our example, we’ll say we’re detecting only photons of the six above energies.
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Pul
se V
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geP
ulse
Vol
tage
TimeTime
230 keV80 keV
120 keV140 keV
30 keV
180 keV
50
75
1
00
1
25
1
50
1
75
2
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5
We’ll let them add up over time.
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230 keV
80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
6
10
15
40
10
2
We’ve stopped our detector, and now we’ll tally up each type of photon detected.
Here are our totals.
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230 keV
80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
6
10
15
40
10
2
Next, we’ll stack up our tally count for each photon on our voltage scale according to its calibrated spot on the scale. 40
30
20
10
Count
s
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230 keV
80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
6
10
15
40
10
2
The colored bars do not represent pulse heights here…
40
30
20
10
Count
s
But rather the total amount of each type of energy detected.
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230 keV
80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
6
10
15
40
10
2
We can see that we have collected mostly 140 keV photons—the type of gamma emission associated with Tc-99m.
40
30
20
10
Count
s
If our source is indeed Tc-99m, why are we getting the other photon energy readings?
This is our This is our photopeakphotopeak because because
it most repeatedly it most repeatedly generated the level of generated the level of scintillation light that scintillation light that
resulted in this resulted in this voltage pulsevoltage pulse
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230 keV
80 keV
120 keV140 keV
30 keV
180 keV
Volts
0 25 50 75 100 125 150 175 200 225 250 275 300
6
10
15
40
10
2
Some explanations for these other gammas detected are …
40
30
20
10
Count
s
Partially detected photons
Back-scattered photons
Compton Scattered photons Primary
gamma photons Extra
electrons emitted from photo-cathode
Two gamma photons detected simulta-neously
The 140 keV primary gamma photons are coming directly from the source. How do we extract them from the others so they can give us some reliable information?
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If we lived in a perfect world with no If we lived in a perfect world with no scatter and perfect detectors, we would scatter and perfect detectors, we would get a gamma “pulse-height” spectrum that get a gamma “pulse-height” spectrum that looked like…looked like…
0 25 50 75 100 125 150 175 200 225 250 275 300
40
30
20
10
Count
s
Volts
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Figure 03: Peak broadening as seen with scintillation detectorsFigure 03: Peak broadening as seen with scintillation detectors
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Pul
se V
olta
geP
ulse
Vol
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TimeTime
How do we extract How do we extract the pulses that the pulses that represent the true represent the true gamma energy of gamma energy of a radionuclide?a radionuclide?
Answer: Pulse Answer: Pulse Height AnalysisHeight Analysis
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Pulse Height AnalyzerPulse Height Analyzer
According to Sodee text:According to Sodee text: “ “The pulse height analyzer is an electronic The pulse height analyzer is an electronic
device that enables the operator to select device that enables the operator to select pulses of a certain height and to reject all pulses of a certain height and to reject all pulses of a different height.”pulses of a different height.”
Paul Early, D. Bruce Sodee, Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear MedicinePrinciples and Practice of Nuclear Medicine, 2nd Ed., , 2nd Ed., (St. Louis: Mosby 1995), pg. 141.(St. Louis: Mosby 1995), pg. 141.
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Single Channel Analyzer (SCA)Single Channel Analyzer (SCA)
A pulse height analyzer that detects only A pulse height analyzer that detects only one set of pulses.one set of pulses.
We will use a Single Channel Analyzer example to We will use a Single Channel Analyzer example to demonstrate how we can separate our 140 keV photons demonstrate how we can separate our 140 keV photons from the photons of other energies.from the photons of other energies.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
00
1
25
1
50
1
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2
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52
25
We’ll go back to our collection of We’ll go back to our collection of pulses over time to see how we can pulses over time to see how we can distinguish the 140 keV pulses from distinguish the 140 keV pulses from
the other pulses representing the other pulses representing detected photons of different detected photons of different
energies.energies.
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Lower Level Discriminator (LLD)Lower Level Discriminator (LLD)
The electronically and arbitrarily The electronically and arbitrarily established threshold that a pulse much established threshold that a pulse much reach in order to be counted as detected.reach in order to be counted as detected.
For our example, we’ll establish a threshold of For our example, we’ll establish a threshold of 10% below the 140 volt pulse (140 keV),10% below the 140 volt pulse (140 keV),
that is, we’re going to electronically tell our that is, we’re going to electronically tell our system NOT to accept any pulses that do not system NOT to accept any pulses that do not
reach 126 volts in height (126 keV).reach 126 volts in height (126 keV).
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
00
1
25
1
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25
Here’s our LLD Here’s our LLD lineline(at 126 Volts)(at 126 Volts)
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
00
1
25
1
50
1
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And here’s its effectsAnd here’s its effects
All pulses less than 126 volts are All pulses less than 126 volts are not seen (counted)not seen (counted)
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Let’s count our pulses Let’s count our pulses and see what we got.and see what we got.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
00
1
25
1
50
1
75
2
00
22
52
25
But Wait! Some of But Wait! Some of these are not 140 volt these are not 140 volt
pulses!pulses!
11
22
3344 55 66
77
88
99
We get nine We get nine pulses countedpulses counted
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Upper Level Discriminator Upper Level Discriminator (ULD)(ULD)
An Upper Level Discriminator is just a second An Upper Level Discriminator is just a second Lower Level Discriminator.Lower Level Discriminator.
It also has an electronic threshold that will only It also has an electronic threshold that will only recognize pulses of an arbitrarily selected recognize pulses of an arbitrarily selected voltage height.voltage height.
The ULD threshold is set above the LLD The ULD threshold is set above the LLD threshold.threshold.
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Let’s set our ULD for 10% above our Let’s set our ULD for 10% above our desired 140 volt (140 keV) pulse height.desired 140 volt (140 keV) pulse height.
This would come to 154 volts (154 keV).This would come to 154 volts (154 keV).
This means all pulses BELOW 154 volts This means all pulses BELOW 154 volts would be NOT be counted.would be NOT be counted.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
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1
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1
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1
75
2
00
50
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1
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Here’s the ULD Here’s the ULD thresholdthreshold
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Pul
se V
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ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
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1
25
1
50
1
75
2
00
50
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1
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And here’s its effectsAnd here’s its effects
Is this what we wanted? Are these the Is this what we wanted? Are these the counts we need? How can we use counts we need? How can we use
this????this????
What the…?What the…?
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Anticoincidence Logic CircuitAnticoincidence Logic Circuit
Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 116., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 116.
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Pulses Pulses from from
AmplifieAmplifierr
LLDLLD
ULDULD
Anti-Anti-coincidenccoincidenc
e logice logic
OutpuOutputt
All of our All of our pulses come pulses come in from the in from the amplifier at amplifier at
their their proportional proportional
voltage voltage heightsheights
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Pulses Pulses from from
AmplifieAmplifierr
LLDLLD
ULDULD
Anti-Anti-coincidenccoincidenc
e logice logic
OutpuOutputt
One copy of One copy of pulses goes to pulses goes to
the LLD.the LLD.
One copy of One copy of pulses goes to pulses goes to
the ULD.the ULD.
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Pulses Pulses from from
AmplifieAmplifierr
LLDLLD
ULDULD
Anti-Anti-coincidenccoincidenc
e logice logic
OutpuOutputt
Only the 140, 180, & 230 V Only the 140, 180, & 230 V pulse copies cross the LLD pulse copies cross the LLD
threshold and are accepted.threshold and are accepted.
Only the 180 & Only the 180 & 230 V pulse copies 230 V pulse copies
cross the ULD cross the ULD threshold and are threshold and are
acceptedaccepted
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Pulses Pulses from from
AmplifierAmplifier
LLDLLD
ULDULD
OutpuOutputt
In the anticoincidence In the anticoincidence logic circuit the copies logic circuit the copies
of the 180 & 230 V of the 180 & 230 V pulses arrive at the pulses arrive at the same time (for they same time (for they
were generated at the were generated at the same time.)same time.)
The copy of the 140 V pulse The copy of the 140 V pulse arrives by itself because its arrives by itself because its
copy broke the LLD copy broke the LLD threshold but not the ULD threshold but not the ULD
thresholdthreshold..
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Pulses Pulses from from
AmplifieAmplifierr
LLDLLD
ULDULD
OutpuOutputt
Because the 180 and Because the 180 and 230 V pulse copies 230 V pulse copies arrived at the same arrived at the same
time (they were time (they were generated at the same generated at the same time) the coincidence time) the coincidence
logic cancels them out.logic cancels them out.
The single 140 V (140 keV) The single 140 V (140 keV) pulse has no copy and pulse has no copy and
survivessurvives
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Pulses Pulses from from
AmplifierAmplifier
LLDLLD
ULDULD
Anti-Anti-coincidenccoincidenc
e logice logic
OutpuOutputt
From all the From all the pulses we pulses we collect one collect one “count” of a “count” of a 140 V pulse 140 V pulse
(140 kev (140 kev photon).photon).
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
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1
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1
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75
2
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50
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In Effect, our Coincidence Circuit In Effect, our Coincidence Circuit enables us to cancel out our enables us to cancel out our unwanted oversized pulses.unwanted oversized pulses.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
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1
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1
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And get only the desired And get only the desired pulses.pulses.
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Pul
se V
olta
geP
ulse
Vol
tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
75
1
00
1
25
1
50
1
75
2
00
50
75
1
00
1
25
1
50
1
75
2
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22
52
25
We go from this….We go from this….
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Pul
se V
olta
geP
ulse
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tage
TimeTime
230 230 keVkeV
80 keV80 keV
120 120 keVkeV140 140 keVkeV
30 keV30 keV
180 180 keVkeV
50
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To this.To this.
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230 230 keVkeV
80 keV80 keV
120 120 keVkeV
140 140 keVkeV
30 keV30 keV
180 180 keVkeV
VoltsVolts
0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300
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22
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Cou
nts
Cou
nts
We end up with an energy “window” that We end up with an energy “window” that discriminates against photon energies that are from discriminates against photon energies that are from
indirect sources.indirect sources.
This is a Single Channel AnalyzerThis is a Single Channel Analyzer
(LLD)(LLD) (ULD)(ULD)
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Fig 2-6 from your Prekeges TextFig 2-6 from your Prekeges Text
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230 230 keVkeV
80 keV80 keV
120 120 keVkeV
140 140 keVkeV
30 keV30 keV
180 180 keVkeV
VoltsVolts
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This is a Single Channel AnalyzerThis is a Single Channel Analyzer
(LLD)(LLD) (ULD)(ULD)
In reality, we add up the counts from the different In reality, we add up the counts from the different photon energies and get something like this…photon energies and get something like this…
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230 230 keVkeV
80 keV80 keV
120 120 keVkeV
140 140 keVkeV
30 keV30 keV
180 180 keVkeV
VoltsVolts
0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300
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Cou
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This is a Single Channel AnalyzerThis is a Single Channel Analyzer
(LLD)(LLD) (ULD)(ULD)
This shows a 20% energy (window) around the 140 keV This shows a 20% energy (window) around the 140 keV photopeak.photopeak.
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Let’s apply our gain control to the real energy spectrumLet’s apply our gain control to the real energy spectrum
GAINGAIN
11
22 55
1010
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GAINGAIN
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GAINGAIN
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GAINGAIN
11
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GAINGAIN
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GAINGAIN
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GAINGAIN
11
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GAINGAIN
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GAINGAIN
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GAINGAIN
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GAINGAIN
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Full Width at Half Maximum Full Width at Half Maximum (FWHM)(FWHM)
A A measurement measurement
of energy of energy resolution—a resolution—a
means of means of showing how showing how
well your well your detector can detector can discriminate discriminate
energy energy differences.differences.
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VoltsVolts
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(X
C
ou
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(X
1
00
0)
10
00
)
Full Width at Half Maximum Full Width at Half Maximum (FWHM)(FWHM)
First…First…
Find point on Find point on scale that scale that
correlates to correlates to your peak your peak
counts.counts.140 V140 V
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Next…Next…
Find the Find the maximum maximum
counts of the counts of the spectrum.spectrum. 140 V140 V
42,000 Counts42,000 Counts
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Then…Then…
Figure out Figure out where ½ the where ½ the
maximum maximum counts counts
intersects the intersects the peak of the peak of the spectrumspectrum
140 V140 V
21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)
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Now…Now…
Determine Determine how the full how the full width of the width of the
photopeak at photopeak at ½ maximum ½ maximum
counts counts translates to translates to
the scale the scale belowbelow
126 V126 V
21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)
158 V158 V
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The FWHM is The FWHM is based on the based on the
following:following: 126 V126 V
21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)
158 V158 V
% Resolution = % Resolution = Upper Scale Reading – Lower Scale ReadingUpper Scale Reading – Lower Scale Reading X 100 X 100 Photopeak scale readingPhotopeak scale reading
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For our system, For our system, our calculations our calculations
would be as would be as follows:follows:
126 V126 V
21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)
158 V158 V
% Resolution = % Resolution = 158 V - 126 V158 V - 126 V X 100 = 23 % X 100 = 23 %
140 V140 V
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126 V126 V
21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)
158 V158 V
% Resolution = % Resolution = 158 V - 126 V158 V - 126 V X 100 = 23 % X 100 = 23 %
140 V140 V
A FWHM of 23 % A FWHM of 23 % actually stinks.actually stinks.
7 or 8 % would 7 or 8 % would be a more be a more
desirable value.desirable value.
This means our This means our photopeak photopeak
should be much should be much slimmer.slimmer.
Our system Our system likely needs likely needs
repair.repair.
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A highly A highly resolute resolute
photopeaphotopeak (with a k (with a
low low FWHM) FWHM)
should be should be skinny.skinny.
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MultiChannel Analyzer (MCA)MultiChannel Analyzer (MCA)
A “digital” means to collect and record A “digital” means to collect and record counts along a set of voltage channelscounts along a set of voltage channels
Uses Analogue to Digital Conversion Uses Analogue to Digital Conversion (ADC) to discern pulse sizes and assign (ADC) to discern pulse sizes and assign them to memory locationsthem to memory locations
Greatly increases the flexibility of selecting Greatly increases the flexibility of selecting and measuring counts from various energy and measuring counts from various energy sourcessources
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MultiChannel AnalyzerMultiChannel Analyzer
Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
Like SCAs, gamma photons generate a number Like SCAs, gamma photons generate a number of pulse sizes along a voltage scale or of pulse sizes along a voltage scale or
“channels.”“channels.”
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
These pulse sizes are converted to a discrete These pulse sizes are converted to a discrete value based on the channel in which they fall. value based on the channel in which they fall. This is called Analogue to Digital Conversion.This is called Analogue to Digital Conversion.
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
In other words, there is a rounding off of pulse In other words, there is a rounding off of pulse sizes so that they equal a digitized amount, sizes so that they equal a digitized amount, such as 2.8 and 3.2 are assigned to digital such as 2.8 and 3.2 are assigned to digital
value “3.”value “3.”
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
Most scintillation detectors now use MCAs Most scintillation detectors now use MCAs to define and discern gamma emission to define and discern gamma emission
spectrumsspectrums
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
The MCA can select digital channels for The MCA can select digital channels for analysis of digitized counts that represent analysis of digitized counts that represent
incident photons energies upon the incident photons energies upon the scintillation detectorscintillation detector
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Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.
The MCA can count from selected multiple The MCA can count from selected multiple channels or can collect a count from all channels or can collect a count from all
channels.channels.
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Multi Channel AnalyzerMulti Channel Analyzer
Calibration—HV should be set so that the same Calibration—HV should be set so that the same energy level (662 keV for Cs-137) is assigned to energy level (662 keV for Cs-137) is assigned to an acceptable range of channels or data bins.an acceptable range of channels or data bins.
Frequent changes to HV to adjust the energy level Frequent changes to HV to adjust the energy level to the channels means that something is amiss.to the channels means that something is amiss. HV supplyHV supply Optic couplingOptic coupling Hermetic sealHermetic seal
Correction factors are applied to channels to Correction factors are applied to channels to relate to other energy levels.relate to other energy levels.
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Multi-Channel AnalyzerMulti-Channel Analyzer
Fig. 2-10 from Prekeges:Fig. 2-10 from Prekeges:
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Figure 09A: Scintillation detector probe geometryFigure 09A: Scintillation detector probe geometry
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Figure 09B: Scintillation detector well geometryFigure 09B: Scintillation detector well geometry
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Thyroid Probe and Well CounterThyroid Probe and Well CounterQuality ControlQuality Control
Daily:Daily:
ConstancyConstancy
Calibration of photopeakCalibration of photopeak
Quarterly:Quarterly:
Chi-SquareChi-Square
Energy Resolution (Sodee)Energy Resolution (Sodee)
Linearity (Sodee)Linearity (Sodee)
Confirm Windows (Sodee)Confirm Windows (Sodee)
Annually:Annually:
Efficiency (Prekeges)Efficiency (Prekeges)Paul Early, D. Bruce Sodee, Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear MedicinePrinciples and Practice of Nuclear Medicine, 2nd Ed., (St. , 2nd Ed., (St.
Louis: Mosby 1995), pg. 149.Louis: Mosby 1995), pg. 149.
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You are hereYou are here